Color cathode-ray tube with in-line plural electron sources and central section of common grid protruding toward central source

ABSTRACT

In a cathode-ray tube of the single-gun, plural-beam type in which the plurality of beams are made to intersect each other substantially at the optical center of a main focusing lens, a beam-generating assembly is provided in which the respective beams are generated at suitable distances from the main focusing lens to correct for any disparity in the focusing effects imparted to the respective beams by the main focusing lens and any auxiliary lens and thereby to cause the beams to be precisely focused on the screen.

Unite States Patent Inventor Appl. No.

Filed Patented Assignee Priority Senri Miyaoka Kanagawa-ken, Japan Oct. 29, 1969 Nov. 9, 1971 Sony Corporation Tokyo, Japan Mar. 7, 1969 Japan COLOR CATHODE-RAY TUBE WITH lN-LlNE PLURAL ELECTRON SOURCES AND CENTRAL SECTION OF COMMON GRID PROTRUDING TOWARD CENTRAL SOURCE 3 Claims, 8 Drawing Figs.

US. Cl 313/70 C, 313/92 Int. Cl H0lj 29/50, l-iOlj 31/20, HOlj 29/32 [50] Field of Search 313/70 C, 69 C [56] References Cited UNITED STATES PATENTS 2,825,847 3/1958 De Gier 313/70 C 3,448,316 6/1969 Yoshida et a1. 313/70CX Primary Examiner-Robert Segal Attorneys-Albert C. Johnston, Robert E. lsner, Lewis H.

Eslinger and Alvin Sinderbrand PATENIEDuuv 9 Ian 3,5191; 86

SHEET 2 OF 2 29/4/1405? 4F A BEA/11 5P0r ON A SCP'f/V f/zzm) -70 oo {00 -20 0 2 40 6 0 7200 T 650V 6/? k k 7 125671 001?? INVENTOR.

2 M 3 64 5 SENRI MIYAOKA ATTORNEY COLOR CATIIODE-RAY TUBE WITH IN-LINE PLURAL ELECTRON SOURCES AND CENTRAL SECTION OF COMMON GRID PROTRUDING TOWARD CENTRAL SOURCE This invention relates generally to cathode-ray or color picture tubes of the single-gun, plural-beam type, and particularly to tubes of that type in which the plural beams are passed substantially through the optical center of a common electron lens by which the beams are intended to be focused on the color phosphor screen.

in single-gun, plural-beam color picture tubes of the type to which this invention relates, for example, as specifically disclosed in U.S. Pat. No. 3,448,316, issued June 3, 1969, and having a common assignee herewith, a plurality of electron beams is emitted or originated by a beam-generating cathode assembly and converged to cross or intersect each other at a location between the cathode assembly and the color screen upon which the beams impinge, and a single main focusing lens for focusing all of the beams on the screen is positioned to dispose its optical center substantially at the location where the beams intersect, whereby the imparting of coma and spherical aberrations to the beams by the main focusing lens is substantially diminished. When the beams are thus converged to intersect each other substantially at the optical center of the main focusing lens, at least certain of the beams emerge from the lens along divergent paths, and pairs of convergencedeflecting plates may be arranged along such divergent paths and have voltages applied thereacross to deflect the divergent beams in directions for causing all of the beams to converge at a common point on the apertured beam-selecting grill or mask associated with the color screen, or the divergent beams may be allowed to land on the beam-selecting grill or mask at spaced locations with suitable time delays being applied to the color signals by which the respective beams are modulated so as to obtain correspondence of the pictures produced on the screen. In either case, the beams are acted upon by the magnetic fields resulting from the application of horizontal and vertical sweep signals to the corresponding coils of a deflection yoke, whereby the beams are made to scan the screen in the desired raster.

In singlegun, plural-beam color picture tubes as described above, the beams pass through the main focusing lens at different respective angles to the axis of the lens, causing the beams to receive different respective focusing effects and thereby causing the beams to be focused at different respective distances from the main focusing lens.

Furthermore, single-gun, plural-beam color picture tubes as described above generally include an auxiliary lens positioned between the beam-generating means and the main focusing lens. Such auxiliary lens is employed to prefocus the beams and sometimes also to converge the beams so that they cross or intersect each other substantially at the optical center of the main focusing lens. In such color picture tubes, the beams pass through the auxiliary lens at different respective distances from the optical axis of the lens, causing the beams to receive different prefocusing effects and thereby also causing a difference between the focusing distances for the respective beams. This disparity in beam-focusing distances causes certain of the beams to impinge upon the screen in a slightly unfocused condition, resulting in unequal beam spot sizes and thus producing pictures having less than perfect resolution.

Accordingly, it is generally an object of this invention to provide "a cathode-ray or color picture tube of the described type having high resolution of the picture on the screen.

More specifically, it is an object of this invention to provide a cathode-ray or color picture tube of the described type in which the beams are focused at substantially the same distance from the main focusing lens, so that such beams will impinge on the screen at sharply defined spots of substantially uniform size.

In accordance with an aspect of this invention, a cathoderay or color picture tube of the single-gun, plural-beam type is provided with a beam-generating cathode assembly for generating a plurality of electron beams in which the beams are originated at locations which are spaced suitable respective distances from the main focusing lens to correct for the different focusing effects imparted to the beams by the lens so that the beams are focused at substantially the same distance from the main focusing lens.

In accordance with another aspect of this invention, a cathode-ray or color picture tube of the single-gun, pluralbeam type having an auxiliary lens is provided with a beamgenerating cathode assembly for generating a plurality of beams in which the beams are originated at locations which are spaced suitable respective distances from the main focusing lens to correct for the different focusing effects imparted to the beams by the main focusing lens and the different prefocusing effects imparted to the beams by the auxiliary lens, so that the beams are focused at substantially the same distance from the main focusing lens.

The above, and other objects, features and advantages of the invention, will be apparent in the following detailed description of illustrative embodiments thereof which is to be read in connection with the accompanying drawings, in which like or corresponding parts are designated by the same reference designation in the various views, and wherein:

FIG. I is a schematic, horizontal sectional view of an existing single-gun, plural-beam color picture tube;

FIG. 2 is a schematic, horizontal sectional view of a portion of another existing single-gun, plural-beam color picture tube;

FIG. 3 is a diagrammatic view illustrating the optical equivalent or analogy of the single-gun, plural-beam color picture tube shown in FIG. 2;

FIG. 4 is a diagrammatic view illustrating the optical equivalent or analogy of a single-gun, plural-beam color picture tube according to an embodiment of this invention;

FIG. 5 is a graph comparing the beam spot sizes of a prior art, single-gun, plural-beam color picture tube and one according to an embodiment of this invention; and

FIGS. 6, 7 and 8 are views similar to FIG. 2, but showing single-gun, plural-beam color picture tubes according to various embodiments of this invention.

In order that the single-gun, plural-beam color picture tubes according to the present invention may be better understood, the principles and features of prior single-gun, plural-beam color picture tubes will first be described in detail with reference to FIGS. I to 3,

Referring initially to FIG. I, it will be seen that a prior art single-gun, plural-beam color picture tube comprises a glass envelope (not shown) having a neck and a cone extending from the neck to a color screen S provided with the usual arrays of color phosphors. Disposed within the neck is a single electron gun including three cathodes K K and K having their respective beam-generating surfaces disposed as shown so that the respective beams B B and B emitted therefrom are directed in a substantially horizontal plane containing the axis of the gun, with the central beam 8,; being coincident with such axis and the side beams B and B, being parallel thereto. A first grid 0 is spaced from the beam-generating surfaces of cathodes K K and K and has apertures h h and h formed therein in alignment with the respective cathodebeam-generating surfaces. A common grid 0, is spaced from the first grid and has apertures h h and it formed therein in alignment with the respective apertures of the first grid. Cathodes K K and K and grids G and G, cooperate to form three beam-generating means. Successively arranged in the axial direction away from the common grid 0 are openended, tubular grids or electrodes 0,, 0. and 0 respectively, with cathodes K K and Kat grids G, and 0,, and electrodes 0,, 0 and 0, being maintained in the depicted assembled positions thereof, by suitable, nonillustrated support means of an insulating material.

For operation of the color picture tube of FIG. 1, appropriate voltages are applied to the grids G and 0, and to the electrodes 0,, G and 0 so as to establish an electron lens field around the axis of electrode 0, to form an auxiliary lens L,, which is indicated by its optical equivalent, and an electron lens field around the axis of electrode G, to form a main focusing lens L,,,, which is indicated by its optical equivalent. Auxiliary lens L prefocuses beams 3 B and B and causes side beams B and B to converge so that they cross or intersect with beam 8 substantially at the optical center of main focusing lens L,,,.

Also included in the color picture tube of FIG. 1 are electron beam convergence-deflecting means C which comprise shielding plates P and P disposed in the depicted spaced relationship at opposite sides of the tube axis, and axially extending deflector plates Q and O which are disposed, as shown, in outwardly spaced, opposed relationship to shielding plates P and P, respectively. Although depicted as substantially straight, it is to be understood that the deflector plates Q and Q may, alternatively, be somewhat curved or outwardly bowed, as is well known in the art.

The shielding plates P and P are equally charged and disposed so that the central electron beam B will pass substantially undeflected between the shielding plates P and P, while the deflector plates Q and Q have negative charges with respect to the plates P and P so that electron beams B and B will be convergently deflected, as shown, by the respective passages thereof between the plates P and Q and the plates P and Q. The screen S is located so that it substantially coincides with the intersection of beams B B and B whereby the beams impinge upon screen S at a common area.

In operation, electron beams B 8 and B which emanate from the beam-generating surfaces of cathodes K K and K will pass through the respective grid apertures h h and h to be intensity-modulated with what may be termed the red," green and blue intensity modulation signals applied between the said cathodes and the first grid G,, The electron beams will then be converged by the auxiliary lens L, to cross each other substantially at the optical center of the main lens L and to emerge from the latter with beams B and B diverging from beam 8 Thereafter, the central electron beam B will pass substantially undeflected between shielding plates P and P since the latter are at the same potential. Passage of the electron beam B between the plates P and Q and of the electron beam 3,; between the plates P and Q will however, result in the convergent deflections thereof as a result of the voltages applied therebetween, and the system of FIG. 1 is so arranged that the electron beams B B and B will desirably converge at a common area on screen S.

Electron beam scanning of the face of the color phosphor screen is effected by deflection yoke D, which receives horizontal and vertical sweep signals whereby the beams are made to scan the screens in the desired raster.

FIG. 2 shows a portion of the electron gun of another prior art, single-gun, plural-beam color picture tube which operates in substantially the same manner as that of FIG. 1, with the exception that the cathodes K K and K are arranged as shown on an arcuate surface whose center substantially corresponds with the optical center of main lens L so that the respective beams B B and B emitted therefrom are directed in a substantially horizontal plane containing the axis of the gun, with the central beam B being coincident with such axis and the side beams B and B converging toward axis so that they intersect substantially at the optical center of main lens L While this eliminates the need for the converging function of auxiliary lens L,, an auxiliary lens L may still be employed to prefocus beams B B and B Further, in the gun of FIG. I, the first grid is constituted by an assembly of three individual grids G suitably arranged about cathodes K K and K to provide uniform spacing between the cathodes and apertures h h and h respectively.

For operation of the color picture tube of FIG. 2, appropriate voltages are applied to the grids G 1 and G and the electrodes G 0, and 6,, so as to establish an electron lens field in grid (l, to form an auxiliary lens L which is indicated by the cquipotential lines in grid 0,, and an electron lens field around the axis of electrode 6, to form a main focusing lens L which is indicated by its optical equivalent. The end surfaces of grid G and electrode G may be suitably shaped so that they are substantially perpendicular to the beams B B and B whereby the electron lens field-forming auxiliary lens L, will be shaped to prefocus the beams and to avoid further convergence of the beams.

Referring to FIG. 3, which is a diagrammatic view illustrating the optical equivalent or analogy of the prior art, singlegun, plural-beam color picture tube shown in FIG. 2, the focusing characteristics of that tube will now be explained. Note that for simplicity only one of the side beams B and B, is shown, as the other would appear symmetrically identical about the axis of the tube.

Central beam B and side beam 8,, or 8,; are shown as originating from their respective optical image points P located on an arcuate line Y concentric with the optical center of main focusing lens L,,, and are shown to converge substantially at the optical center of main focusing lens L,,.,. Side beam 3,; or 8,, passes through auxiliary lens L, at a distance from its optical axis while central beam B passes through lens L, substantially along the optical axis. This causes a different prefocusing effect to be imparted to side beam B or 8 from that imparted to central beam B Specifically, side beam 8,, or B is prefocused to a greater degree than central beam B Furthermore, side beam 8,; or 8,, passes through main focusing lens L,,, at an angle to the optical axis of the latter while central beam B passes through it substantially along the optical axis. This causes a different focusing effect to be imparted to side beam E or B from that imparted to central beam B Once again, side beam B or B is focused to a greater degree than central beam B This results in side beam B or B being focused closer to main lens L,, than central beam B If a screen S is located at the focus point of side beam 8,; or B central beam B would impinge upon screen S in a slightly unfocused condition and would thus produce a larger spot than that produced by side beam 8,; or B Similarly, if screen S was moved to location S, corresponding to the focus point of central beam B side beam B or B would impinge on screen 5' in a slightly unfocused condition and would thus produce a larger spot than that produced by central beam B Thus the disparity in beam-focusing distances would produce a picture having less than perfect resolution.

Of course, a similar analysis of the tube shown in FIG. 1 would produce a similar result, with the only difference being that side beam B or B would appear parallel to central beam 8 between its image point P and auxiliary lens L,.

FIG. 4 shows in solid lines the optical equivalent or analogy of a single-gun, plural-beam color picture tube according to this invention, superimposed upon the optical equivalent or analogy of the prior art tube as shown in FIG. 3, indicated in dashed lines. Screen S is shown located at the focusing point B; of side beam B or B Central beam B of the prior art tube is focused at focusing point 8,, which is shown on the tube axis a distance AB beyond screen S. According to the present invention, the optical image point P' of central beam B is located a distance from main focusing lens L that is greater by AA than the distance A from the optical image point P of the central beam B to the lens L,,, in the prior art tube, in order to increase the focusing effects imparted to central beam B Selection of a suitable distance results in the coincident focusing of side beam B or B and central beam B a at focus point B and thus produces minimum beam spot sizes upon screen S, thereby improving the resolution of the picture produced by the tube.

Referring to FIG. 4, a mathematical analysis of the focusing effects imparted to central beam B of the prior art tube results in the following equation:

1f) in which A=distancc between optical image point P and main focusing lens L,,,',

B=distance between main focusing lens l.,, and focusing point B and f=effective focal length of lens L,,, and L,. A similar mathematical analysis of the focusing effects imparted to central beam B of the tube according to the present invention results in the following equation:

in which A, B andfrepresent the same quantities as in equation 1) and AB=distance between focusing point B and focusing point 8,; and

AA=distance between optical image point P and optical image point P. In accordance with equations (1) and (2), it can be demonstrated that locating the beam source of central beam 8,; a farther distance from main lens L,,, than the beam sources of side beams B and B will produce coincident focusing upon screen S.

Referring to FIG. 5, which is a graph comparing the beam spot sizes of a prior art, single-gun, plural-beam color picture tube and one according to an embodiment of this invention, the beneficial results of the present invention will become apparent. The vertical axis of the graph of FIG. 5 corresponds to the diameter of the beam spot on the screen, and the horizontal axis corresponds to the voltage applied to electrode G, which is related to the power of main focusing lens L,,,. Curve 1 represents the beam spot size of central beam B in a prior art tube as hereinbefore described. Curve 2 represents the beam spot size of side beams B and B,, of both the prior art tube and the tube according to this invention. Since curves 1 and 2 reach minimums at different locations, it is clearly shown that there is no voltage which may be applied to electrode G, to simultaneously produce minimum, identical beam spot sizes for the respective beams.

Curve 1 represents the beam spot size of central beam B in a tube according to this invention. Since curve 1' is substantially similar to curve 2, the tube according to this invention substantially attains the optimal focusing condition since curves 2 and 1 reach minimums at substantially identical electrode G, voltages. The voltage applied to electrode G, may therefore be readily adjusted to simultaneously produce identical, minimum beam spot sizes for the respective beams, thereby producing a picture having higher resolution than that of the prior art tube.

Examples of the structure of the electron gun portions of a color picture tube in accordance with this invention will now be described with reference to FIGS. 6, 7 and 8.

FIG. 6 illustrates the application of this invention to a single-gun, plural-beam color picture tube of the type shown in FIG. I, and in which the cathodes K K and K are aligned in parallel so that the beams generated therefrom will be substantially parallel. Central cathode K is spaced from main lens L,,, by a distance suitably greater than side cathodes K,, and K,, to correct for the disparity in focusing distances of the prior art tube as previously described. First grid G, contains a protuberant portion 11 suitably shaped so that the distance from cathode K to aperture h is substantially identical to the distance from cathodes K and K,, to apertures h and h,,,. Similarly, grid G, contains a protuberant portion 12 suitably shaped so that the distance from aperture h to aperture h is substantially identical to the distance from apertures h and h,,, to apertures h and h By so spacing the respective cathodes and grids, central beam B is substantially identical to side beams B and 8,, but originates from a beam source which is located a suitable distance farther from main lens L,,, than the beam sources of side beams B and B,,.

For operation of the color picture tube of FIG. 6, appropriate voltages, similar to those applied to the prior art tube of FIG. I, are applied to the grids G, G and t the electrodes 6,, G, and G so as to establish an electron lens field around the axis of electrode G, forms a main focusing lens L indicated by its optical equivalent, and further to establish an electron lens field which forms the auxiliary lens L, within the end portion ofelectrode G, adjacent grid G and which is also indicated by its optical equivalent. The protuberant portion 12 of grid G so distorts the electron lens field produced between grid G, and electrode G, as to provide, in effect, a subfocusing lens 13, indicated by its optical equivalent, located within protuberant portion 12 to act only on central beam B The effect of the subfocusing lens 13 resulting from the protuberant portion of grid G is to enhance the focusing effect on central beam B and thereby relatively reduce the distance 01A by which the distance from the central beam source to the main lens L,, has to be greater than the distance from the side beam sources to the main lens in order to achieve precise focusing of all of the beams on the tube screen. Thus, the distance aA may be made less than that calculated from equation (2) above when the grid G is shaped as shown on FIG. 6 for increasing the focusing effect of the lens field between grid G, and electrode G, on central beam B as compared with its focusing effect on side beams B and B,,. In addition to prefocusing the beams, as described above, the auxiliary lens L, of the embodiment shown in FIG. 6 causes beams B and B to coverage and thereby cross beam H at the optical center of main lens L,,, in the same manner as the prior art tube of FIG. 1.

FIGS. 7 and 8 show applications of this invention to singlegun, plural-beam color picture tubes of types similar to the prior art tube shown in FIG. 2, and in which the cathodes I(,,, K,; and K are arranged so that the beams generated therefrom cross or intersect substantially at the optical center of main lens L,,,, thus eliminating the need for the converging function of auxiliary lens L, In FIGS. 7 and 8, it will be seen that central cathode K,; is again spaced a suitable distance farther from main lens L,,, than side cathodes K and K to correct for the disparity in focusing distances of the prior art tube as previously described. The first grid is constituted by an assembly of three individual grids G, suitable arranged about cathodes K K and K,, to provide uniform spacing between the cathodes and the respective apertures of the first grids. Second grid G contains a protuberant portion 12 suitably shaped so that the distance from the central first grid to protuberant portion 12 is substantially identical to the distance from the side first grids to side portions of grid G By so spacing the respective cathodes and grids, central beam 8 is substantially identical to side beams B and B but originates from a beam source which is located a suitable distance farther from main lens L,, than the beam sources of side beams B and B,,.

For operation of the color picture tubes of FIGS 7 and 8, appropriate voltages, similar to those applied to the prior art tube of FIG. 2, are applied to the grids G, and G, and the electrodes 6,, G, and G so as to establish an electron lens field around the axis of electrode G., to form a main focusing lens L,,,, which is indicated by its optical equivalent, and further to establish an electron lens field 14 between grid G, and electrode G, which forms the auxiliary lens similar to the previously mentioned auxiliary lens L, but acting in this case only to prefocus the three beams. By reason of the protuberance 12 at the center of grid G the focusing effect of lens field 14 near to the optical axis, and hence acting on central beam B is enhanced as compared with the focusing effect of the portions of lens field l4 traversed by beams B and H Such enhanced focusing effect of lens field 14 on central beam 8,; compensates in part for the different focusing effects resulting from the fact that beams B and 8,, pass through field l4 and main focusing lens L, at angles to the optical axis whereas beam 8,; passes therethrough along the axis. Once again the relatively greater distance from the source of beam B to main lens L,, and the enhanced prefocusing effect of lens field 14 on beam B combine to achieve precise focusing of all beams at the screen of the cathode-ray tube.

The end surface of electrode G, may have various shapes, for example, as in FIG. 7 where it is shaped so that it is substantially perpendicular to each of the beams B and B,,, or as in FIG. 8 where it is shaped so that it is substantially perpendicular to the axis of the tube, whereby the electron lens field 14 may have a particular desired configuration so that it will impart appropriate prefocusing effects to the respective beams.

Although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention.

What is claimed is:

l. A color cathode-ray tube comprising a display screen, beam-producing means including a plurality of electron beam sources for respectively producing a central beam directed toward said screen along the axis of the tube and two side beams which originate at opposite sides of the central beam and are directed toward said screen along paths converged with respect to said axis for intersecting said central beam at a point in said tube between said beam-producing means and said screen, said beam sources including three individual cathodes having electron emitting surfaces and a first grid assembly disposed adjacent to said cathodes and having respective grid portions with three apertures therein corresponding to said cathodes, said grid assembly being shaped so that the distances from said cathodes to their respective grid portions are substantially identical, said screen being scanned by said beams and including arrays of difi'erent color phosphors for excitation by said central and side beams, respectively, and electron lens means common to all of said beams and including main focusing lens means for producing an electric focusing field which is substantially centered at said point where the beams intersect and auxiliary lens means positioned between said beam-producing means and said main focusing lens means for at least prefocusing all of said beams, said lens means having different focusing effects with respect to said central beam and said side beams, respectively, said beam source of the central beam being disposed at a greater distance from said main focusing lens means than on said beam sources of the side beams, to compensate for said different focusing effects and thereby cause all of said beams to be precisely focused upon said screen, said electron lens means comprising a second grid disposed adjacent to said first grid assembly and having three apertures corresponding to the three apertures of said first grid assembly, said second grid including a central cylindrical protruding portion having a flat face opposing the central grid portion of said first grid assembly and containing the central aperture of said apertures in the second grid, the distances from the apertures in said first grid assembly to the respective apertures in said second grid being substantially identical, and a plurality of electrodes, said electrodes and said first and second grids being at different electrical potentials to establish the electrical fields constituting said main focusing lens means and said auxiliary lens means with said protruding portion of the second grid causing the field corresponding to said auxiliary lens means to enhance the prefocusing effect thereof on said central beam.

2. A color cathode-ray tube according to claim 1, in which said beam sources are aligned in parallel so that said side beams issue from the respective beam sources substantially parallel with said central beam, and said auxiliary lens means is operative to deflect as well as prefocus said side beams for directing the latter along said paths converged with respect to said axis.

3. A color cathode-ray tube according to claim I, in which said beam sources for the side beams are arranged at angles to said beam source of the central beam so that said side beams issue from the respective beam sources along said paths converged with respect to said axis. 

1. A color cathode-ray tube comprising a display screen, beamproducing means including a plurality of electron beam sources for respectively producing a central beam directed toward said screen along the axis of the tube and two side beams which originate at opposite sides of the central beam and are directed toward said screen along paths converged with respect to said axis for intersecting said central beam at a point in said tube between said beam-producing means and said screen, said beam sources including three individual cathodes having electron emitting surfaces and a first grid assembly disposed adjacent to said cathodes and having respective grid portions with three apertures therein corresponding to said cathodes, said grid assembly being shaped so that the distances from said cathodes to their respective grid portions are substantially identical, said screen being scanned by said beams and including arrays of different color phosphors for excitation by said central and side beams, respectively, and electron lens means common to all of said beams and including main focusing lens means for producing an electric focusing field which is substantially centered at said point where the beams intersect and auxiliary lens means positioned between said beam-producing means and said main focusing lens means for at least prefocusing all of said beams, said lens means having different focusing effects with respect to said central beam and said side beams, respectively, said beam source of the central beam being disposed at a greater distance from said main focusing lens means than on said beam sources of the side beams, to compensate for said different focusing effects and thereby cause all of said beams to be precisely focused upon said screen, said electron lens means comprising a second grid disposed adjacent to said first grid assembly and having three apertures corresponding to the three apertures of said first grid assembly, said second grid including a central cylindrical protruding portion having a flat face opposing the central grid portion of said first grid assembly and containing the central aperture of said apertures in the second grid, the distances from the apertures in saId first grid assembly to the respective apertures in said second grid being substantially identical, and a plurality of electrodes, said electrodes and said first and second grids being at different electrical potentials to establish the electrical fields constituting said main focusing lens means and said auxiliary lens means with said protruding portion of the second grid causing the field corresponding to said auxiliary lens means to enhance the prefocusing effect thereof on said central beam.
 2. A color cathode-ray tube according to claim 1, in which said beam sources are aligned in parallel so that said side beams issue from the respective beam sources substantially parallel with said central beam, and said auxiliary lens means is operative to deflect as well as prefocus said side beams for directing the latter along said paths converged with respect to said axis.
 3. A color cathode-ray tube according to claim 1, in which said beam sources for the side beams are arranged at angles to said beam source of the central beam so that said side beams issue from the respective beam sources along said paths converged with respect to said axis. 